Voltage transfer switch



Jan. 31,1961 w. F. HORTON ET AL 2,970,227

VOLTAGE TRANSFER SWITCH Filed April 50, 1957 I .Z'Z z-a-J /a 24 I /4 A jAwa 7 4040i Ill/11w; Alfluzo, Jet, MAL/1M A #0,? MM,

@6 1 %jMW United Sfttes Patent VOLTAGE TRANSFER SWITCH William F.Horton, Pacific Palisades, and Samuel H.

Auld, Jr., Canoga Park, Califl, assignors to Lear, Incorporated FiledApr. 30, 1957, Ser. N0. 655,982

3 Claims. (Cl. 307-885) This invention relates to voltage controldevices, and more particularly to a voltage transfer switch forconnection between first and second points in a circuit to control theappearance at the second point of voltages produced at the first point.

In many applications, it is important that voltages developed at onepoint in a circuit be controllably introduced into another part of thecircuit. One example is found in aircraft, Where flight error indicatingsignals in the output of a command receiver are applied to the input ofan autopilot. When the autopilot input is first connected to the outputof the receiver, the existing error signal may, for example, indicatethat a sizeable turn is necessary to wipe out the error. If this voltagewere applied directly to the autopilot, the plane might be turnedsharply and create stresses which would cause substantial structuraldamage. To avoid such consequences, it is necessary that the voltageapplied to the autopilot should build up gradually, whereby the planecan be put into the requisite turn smoothly. Presently knownarrangements for accomplishing this purpose are characterized by movingparts; potentiometer means, such as a manually operated mechanicaldash-pot or automatically operated motor-driven potentiometer, areusually employed. Such parts are of course subject to wear and tear andrequire frequent attention and maintenance to maintain a safe level ofreliability. Furthermore, such equipment unduly taxes space and weightrequirements.

It is an object of this invention to provide an improved voltagetransfer switch having no moving parts.

It is another object of this invention to provide an improved voltagetransfer switch for controlling the appearance at an input circuit ofvoltage in a preceding output circuit, which employs a minimum number ofcomponent parts of simple design, and which is smaller in size andlighter in weight than voltage transfer devices of the prior art.

Still another object of this invention is to provide an improved voltagetransfer switch employing semiconductors, wherein parts are not subjectto wear and tear, and which requires a minimum of repair and replacementof parts.

The above and other objects and advantages of this invention will becomeapparent from the following description, taken in conjunction with theaccompanying drawing, in which preferred embodiments are illustrated byway of example. The scope of the invention is pointed out in theappended claims. In the drawing,

Fig. 1 is a schematic diagram of an improved voltage transfer switchemploying a pair of transistors, in accordance with this invention, and

Fig. 2 is a schematic diagram, similar to Fig. l, of a voltage transferswitch employing a single transistor, further in accordance with thisinvention.

Briefly, this invention comprises transistor means connected between theoutput of one stage and the input of a succeeding stage, with a timeconstant or delay network connected to the base circuit of thetransistor means.

A D.-C. voltage is applied to the delay network, which effects anexponential increase in current flow through the transistor means, andhence causes a gradual build-up at the input circuit of the succeedingstage of the voltage existing at the output of the preceding stage.

Referring to Fig. 1, a first pair of terminals 10, 12 represent theoutput terminals of one stage of an electrical circuit, and a secondpair of terminals 14, 16 represent the input terminals to a succeedingstage. A load 18 connected between terminals 14, 16 represents the inputimpedance to the succeeding stage. A pair of n-p-n junction transistors20, 22 have their collector electrodes 24, 26 connected to respectiveterminals 10, 14; terminals 12, 16 are directly connected. The emitterelectrodes 28, 29 of the transistors 20, 22 are each connected to apoint of reference or ground potential. The base electrodes 30-, 32 aredirectly connected, and their junction 34 is connected through a seriesconnected resistor 36 and capacitor 38 to ground. Resistor 36 is aconventional current limiting device, to prevent excessive currentsbeing conducted by the transistors which might damage them.

A source of D.-C. voltage, illustrated as a battery 40, has its positiveterminal connected to a single-throw switch 42, and a resistor 44 isconnected between switch 42 and the junction 46 of resistor 36 andcapacitor 38. A grounded resistor 48 is connected to the switch side ofresistor 44, and a unidirectional conductive device, shown as a diode50, shunts resistor 44.

When switch 42 is closed, capacitor 38 charges exponentially, and hencethe D.-C. voltage at the bases 30, 32 increases exponentially. Since thedynamic impedance of each transistor decreases as the D.-C. voltageapplied to its base increases, the conductivity of the transistorsgradually increases from zero to a maximum. Assuming that a signalvoltage exists across terminals 10, 12 when switch 42 is closed, avoltage appearing between terminals 14, 16 builds up gradually from zeroto a maximum corresponding to the magnitude of the signal voltage. Thetime for this build-up is governed by the time constant of capacitor 38and resistors 36 and 44.

When switch 42 is opened, capacitor 38 discharges through diode 50 andresistor 48. To permit rapid discharge, resistor 48 is much smaller thanresistor 44; however, resistor 48 must be sufficiently large so as notto overheat while it is connected across source 40. This quick-dischargeprovision allows the load 18 to be quickly disconnected from the sourceof signals upon opening switch 42.

The back-to-back connections of the collector and emitter electrodes inFig. l compensates for the unsymmetrical dynamic impedances of typicalsingle transistors to current flow in opposite directions. It will beappreciated that the alternating voltage appearing between terminals 14,16 will resemble the signal voltages presented at terminals 10, 12 onlyif the impedances to positive and negative current excursions aresubstantially equal. Two transistors having substantially equalcharacteristics, and connected back-to-back as shown in Fig. 1,accomplish the desired result.

One voltage transfer switch as above described employs the followingcomponents:

Battery 40 28-volts. Resistor 36 20,000 ohms. Resistor 44 20,000 ohms.Capacitor 38 microfarads. Resistor 48 1,000 ohms.

A voltage transfer switch constructed with components having thesevalues allows the build-up of voltage at ter- 0 minals 14, 16 to occupya period of approximately one second.

Fig. 2 illustrates a voltage transfer switch employing a singletransistor 20'. From the foregoing discussion, it will be apparent thatthis transistor should present the same impedance to current fiow inopposite directions. Transistors of this type are currently beingdeveloped, and this invention embraces the use of such in a voltagetransfer switch.

Diode 50 in Fig. 2 is shown connected in the reverse direction to thatin Fig. 1. This arrangement insures that capacitor 38 will be quicklycharged, and gradually discharged.

Although n-p-n junction transistors have been described and illustratedfor use in this invention, it will be apparent that pen-p junctiontransistors could be employed. In such case, it would be necessary onlyto connect switch 42 to the negative terminal of battery 49. Further, itwill be apparent that the emitter-collector connections could bereversed, in which case the ground connection preferably would be madeto terminals 12, 16.

What is claimed is:

1. A voltage transfer switch comprising first and second transistorseach having emitter, collector and base electrodes, said base electrodesbeing directly connected, a first resistor and a capacitor connected inseries between the junction of said base electrodes and a point ofreference potential, a switching device, asecond resistor connectedbetween said switching device and the junction of said first resistorand said capacitor, a resistive connection between the junction of saidswitching device and said second resistor to said point of referencepotential, at unidirectionally conductive device shunting said secondresistor, first and second pairs of terminals each having a respectiveload impedance connected therebetween, said emitter and collectorelectrodes being connected in series between respective terminals ofeach pair so that the emitter-collector paths of said transistor are inback-toback relation, and means to provide D.-C. path from said emitterelectrodes to the point of reference potential.

2. A voltage transfer switch as defined in claim 1, wherein theunidirectionally conductive device is connected in the forward directionbetween the junction of said first resistor and capacitor and thejunction of said switch and said second resistor.

3. The voltage transfer switch as defined in claim 1, wherein theunidirectionally conductive device is connected in the forward directionbetween the junction of said switching device and said second resistorand the junction of said first resistor and capacitor.

References Cited in the file of this patent UNITED STATES PATENTS2,584,990 Dimond Feb. 12, 1952 2,628,330 Williams Feb. 10, 19532,703,368 Wrathall Mar. 1, 1955 2,763,832 Shockley Sept. 18, 19562,823,322 Trousdale Feb. 11, 1958 2,899,571 Myers Aug. 11, 1959 OTHERREFERENCES Pub. I Junction Transistors Used as Switches, by

Bright, A.I.E.E. Transactions, part I Communications and Electronics,vol. 74 March 1955, pages 119-120.

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